Data Communication Morse, Budot Code History

The History of Data Communications Signals Perhaps the earliest example of digital signals being used to transfer information over long distances dates back to the Greek and Roman signal fires. This was a purely binary system, signaling the occurrence or non-occurrence of predefined events. We have all heard of the use of jungle drums by African tribes and smoke signals used by North American Indians. These were also digital systems, essentially. They conveyed information by changing the length of time between beats or puffs of smoke. In each of these cases the basic requirements of a communications system are met. Senders, receivers and medium agree on methods of encoding and coding the information. Telegraphy was the earliest method of serial data communications using electrical methods. Again, the system included senders, receivers and a medium, as well as an encoding system called the Morse code (shown on one of the next slides). The terms “mark” (signifying a logic1) and “space” (signifying a logic 0) were coined as a result of early attempts to automate the telegraph. This system drew lines on a strip of paper moving under the armature attached to the receiving coil. The teletypewriter was a later development that further automated the transmission and reception of data using electromechanical systems operated by synchronous motors. The arrival of the telephone, designed for voice communication, prompted the development of techniques to transmit and receive data over this existing medium. Eventually networks emerged, which attempted to link multiple transmitter/receivers using a common medium and allowing access to data from many sources.

The Telegraph The telegraph was the first digital communications system. It employed DC batteries to produce current flow in an electrical loop that energised electromagnet coils at both the source and destination ends of each link. When no messages were being sent the loop was held in the “idle” state, (considered a mark, or logic 1) by a latched switch. When this switch was opened the electromagnets at both ends de-energised, alerting the receiving operator that a message would be coming in. A receiving operator could also open his switch during the reception of a message, alerting the sender that, either the line was open or the receiving operator had a priority message to send. This operation was called a break. During data transmission a momentary switch, called the key, was used to send pulses of current to the receiving end. Each time the current was interrupted the electromagnet coil would release an arm creating an audible click. Sending short clicks (dots) and long clicks (dashes) transferred the binary information. The encoding system was based on the Morse code. Telegraph operators became very adept at picking up the digital codes sent from the other end of the link, to the extent that they could recognize the sender by his personal keying speed and style.

Character Codes. Over the years, a number of codes were developed to represent sets of characters. These characters/symbols may be just alpha characters, numbers, and special characters. The number of bits used in a code, determines the number of symbols that can be represented by the code. For instance, since the codes are going to be in binary, the values of each bit can only be 1 or 0, a two value. So, the power of 2 used determines how many symbols it represents: 5 bits = 32 - BAUDOT Code 7 bits = 128 - ASCII Code 8 bits = 256 - Extended ASCII and EBCDIC In today's world the codes used are usually Extended ASCII and EBCDIC. Both are 8 bit codes, and can represent a maximum of 256 characters. ASCII = American Standard Code for Information Interchange. EBCDIC = Extended Binary Coded Decimal Interchange Code. EBCDIC code is used in many IBM computers and in frames being transmitted on LANs. Extended ASCII is the first code that allowed us to represent enough characters to include graphic characters in the code. Extended ASCII code was the break needed to actually represent things such as the boxes and colors that you see at this time, each character or symbol you see is made up of 8 binary bits. The combinations of bits represent different characters/symbols to the computer. The EBCDIC and ASCII codes do not have the same bit configuration to represent the same character.

The Morse Code Morse did not invent the first telegraph system to be put into practical use. That honor belongs to two Britons--Sir Charles Wheatstone, a physicist and inventor, and Sir William Fothergill Cooke, an electrical engineer--who installed the first railway telegraph system in England in 1837, the same year Samuel Morse invented the first American telegraph. However, their system was not a simple one. It was based on five wires which deflected a magnetic needle onto a receiver to indicate the letters of the alphabet. In contrast to this, Morse's system was simpler. It used a single wire for transmitting the signals, and instead of a "deflecting magnetic needle" for receiving the signals, it used an "electromagnet" that attracted a small armature when an incoming signal was received. This made it more reliable. Another interesting feature about Morse's invention was that the incoming signals could be recorded on a moving strip of paper, although this wasn't used for many years, since operators could read incoming messages from the clicking of the receiver. The telegraph employed a coding system called the Morse code. The Morse code is a variable length code that consists of “dots” and “dashes”, a dot being a short duration of a space and a dash being a longer duration. The Morse code uses thirty-eight different

codes to represent all the upper case alphabetic characters, two punctuation symbols (comma and period) and the ten numeric characters.

The Morse Code Chart Morse invented the code he used to send his historic message in 1838. Like the binary system used in modern computers, it is based on combinations of two possible values--in the case of Morse code, a dot or a dash. However, unlike the character codes used in modern computers, the combinations of the two values used to represent characters in Morse code vary in length The advantage of a variable length code can be seen from the table above. By using shorter codes for characters used more often in typical messages, the efficiency of the data transfer could be improved. Notice the letter “E”, a very common character, is represented by a single “dot”. A single “dash” represents the letter “T”. American Morse code is the first and original Morse code character set. Character sets adapted to other languages were developed later. American Morse Code: A. B ... C.. . D .. E. F. . G . H.... I..

J.. K . L_ M N . O. . P..... Q.. . R. ..

S... T U.. V... W. X. .. Y.. .. Z... . 0 __

1 . __ . 2 . . __ . . 3 . . . __ . 4 . . . . __ 5 6...... 7 .. 8 .... 9 . . __

Continental Morse Code or International Morse Code is a widely used de-facto standard. This table summarizes the Western European usage of Continental Morse Code:

The Baudot Code

The Baudot code is a 5-bit code invented in 1874 by Maurice Baudot. The main advantage of the Baudot code over previous codes such as the Morse code was its fixed length. Electromechanical and electronic systems can more easily use a fixed length code for serial data transmission and reception because the length of each character is predictable. The disadvantage of the Baudot code is the limited number of characters it can represent. Using 5 binary bits a total of 32 characters can be represented. As we have seen, with 26 alphabetic characters (in both upper and lower case), ten numeric characters and additional punctuation and control codes, 32 binary codes is insufficient. Implementation of the Baudot code in Teletype applications partially solved this problem by designating two of the codes as FIGURE and LETTER shift characters. This effectively provides for two different codes: one consisting of mostly numerals and symbols and the other consisting of mostly alphabetic characters. When the FIGURE code is sent, all codes after it are interpreted according to the FIGURE code set. When the LETTER code is sent, all codes after are interpreted according to the LETTER code set. Although this system slows down the data throughput, it allows a 5-bit code to represent 62 different characters.

The Baudot Code Chart Notice the two different columns, Letters Shift and Figures Shift, indicating the characters represented by the same code. For example, if the code 1 1111 is sent, the mechanism of a Teletype would physically move so that subsequent codes would produce characters shown under the Letters Shift column. The code 0 0111 would produce the letter “U”. The same code, sent after the Figure Shift code (1 1011), would cause the numeral “‘7” to be printed.

The ASCII Code

The American Standard Code for Information Interchange is the most common code used in the western world. The details of this standard (for use in the United States) are given in the ANSI (American National Standards Institute) standard X3.4-1977. A CCITT standard, (ISA#5) specifies the ASCII code for use in the United Kingdom and other countries. The main difference between these standards is in the designation of the code for the currency symbol. ASCII uses a 7-bit code to produce 128 unique binary patterns. Upper and lower case alphabetic characters, numerals, punctuation marks and symbols, and 34 control codes are represented. The control codes are typically used for device control and information transfer control and are not printable using conventional terminal or printer software. (When troubleshooting a communication system that uses nonprintable characters special software may be needed to display these codes.). An extended version of the ASCII code is sometimes used. This version uses 8 data bits and incorporates 128 additional unique codes for a total of 256 unique codes. Most of the additional codes are used to implement graphics characters.

The ASCII Code Chart The code chart shown below provides a simple method of determining the binary code (or hexadecimal representation of it) for each character represented by the code. Notice that the row of binary numbers across the top of the chart show the three most significant bits (MSB) of the code, while the column of binary numbers on the left side shows the four least significant bits (LSB) of the code.

To determine the code for any given character:

• • •

find the character on the chart move up the column to find the three MSB move across the column to find the 4 LSB

Example: Find the ASCII code for the upper case letter “D” The 3 MSB at the top of the column are 100 (4 hex) The 4 LSB at the left side of the row are 0100 (4 hex) The ASCII code is: 100 0100 (44 hex) Notice that the only difference between upper case letters and lower case letters is that the second most significant bit changes. When the SHIFT key is depressed on a keyboard, this bit is toggled to a zero to produce an upper case character. Similarly, to produce a control code (shown in the first two columns of the chart), the control key is held down while typing a letter key. The two most significant bits are changed to zeroes in the case, sending the ASCII code for a control character.

Extended Binary Coded Decimal Interchange Code (EBCDIC) EBCDIC is an extension to 8 bits of BCDIC (Binary Coded Decimal Interchange Code), an earlier 6-bit character set used on IBM computers. EBCDIC was [first?] used on the successful System/360, anounced on 1964-04-07, and survived for many years despite the almost universal adoption of ASCII elsewhere. Was this concern for backward compatibility or, as many believe, a marketing strategy to lock in IBM customers? IBM created 57 national EBCDIC character sets and an International Reference Version (IRV) based on ISO 646 (and hence ASCII compatible). Documentation on these was not easily accessible making international exchange of data even between IBM mainframes a tricky task. US EBCDIC uses more or less the same characters as ASCII, but different code points. It has non-contiguous letter sequences, some ASCII characters do not exist in EBCDIC (e.g. square brackets), and EBCDIC has some (cent sign, not sign) not in ASCII. As a consequence, the translation between ASCII and EBCDIC was never officially completely defined. Users defined one translation which resulted in a so-called de-facto EBCDIC containing all the characters of ASCII, that all ASCII-related programs use. Some printers, telex machines, and even electronic cash registers can speak EBCDIC, but only so they can converse with IBM mainframes. For an in-depth discussion of character code sets, and full translation tables, see Guidelines on 8-bit character codes.

Chart of character codes.

Here is a simple translation table: Least significant nibble -> 0123456789ABCDE F 0 ... controls ... 1 2 3 ... controls ... 4 â ä à á ã å ç ñ ¢ . < ( + | 5 & é ê ë è à î ï ì ß ! $ * ) ; ^ 6 - / Â Ä À Á Ã Å Ç Ñ ¦ , % _ > ? 7 ø É Ê Ë È Í Î Ï Ì ` : # @ ' = " 8 Ø a b c d e f g h i « » ð ý þ ± 9 ° j k l m n o p q r ª º æ ¸ Æ ¤ A µ ~ s t u v w x y z ¡ ¿ Ð [ Þ ® B ¬ £ ¥ · © § ¶ ¼ ½ ¾ Ý ¨ ¯ ] ´ × C { A B C D E F G H I  ô ö ò ó õ D } J K L M N O P Q R ¹ û ü ù ú ÿ E \ ÷ S T U V W X Y Z ² Ô Ö Ò Ó Õ F 0 1 2 3 4 5 6 7 8 9 ³ Û Ü Ù Ú E.g. the EBCDIC code for "A" is hexadecimal "C1".